Are low-carb diets here to stay? Here’s the truth about how carbohydrates affect your body and why some doctors are embracing the concept...

For years, we’ve been told that eating fat would make us chubby and lead to diabetes, heart disease, obesity and blood disorders.

Now we’re supposed to forget all that.

Some top nutritional scientists are saying that a low-carb diet is really the key to reversing some of the nation’s biggest health problems.

“If Americans eliminated sugary beverages, potatoes, white bread, pasta, white rice and sweet snacks, we’d wipe out almost all our weight and metabolic disease problems,” says Walter Willett, MD, chairman of the nutrition department at Harvard School of Public Health, and author of Eat, Drink and Be Healthy (Free Press).

Americans eat an average of 250-300 grams of carbs a day, accounting for about 55% of our caloric intake.

That percentage has increased in the past 30 years, partly because the government and medical experts encouraged us to cut fat.

At the same time, however, levels of obesity, type 2 diabetes and heart disease increased.

That’s because “the overemphasis on reducing fat caused carbohydrates and sugar consumption to soar,” says Frank Hu, MD, professor of nutrition and epidemiology at Harvard School of Public Health. “That shift is linked to the biggest health problems in America today.”

To manage blood sugar, the pancreas produces insulin, a hormone that moves glucose into the cells to be stored as fuel, or glucagon.

But over time, our bodies can get tired of processing high carb loads, changing the body’s insulin response, says nutritional biochemist Stephen Phinney, MD, a nutritional biochemist and emeritus professor of medicine at the University of California, Davis who has studied carbohydrates for 30 years.

Our bodies become more resistant to insulin, so the pancreas needs to make more to push the same amount of glucose into cells, Dr. Phinney says.

Insulin resistance is the first sign of metabolic syndrome, a red flag for diabetes and heart disease.

About one quarter of U.S. adults have metabolic syndrome, Dr. Phinney says. It’s diagnosed when someone has at least two or three of these symptoms:

High blood triglycerides (over 150 mg)

High blood pressure (over 135/85)

A waist circumference of more than 35 inches in women and 40 inches in men

Low HDL, the “good” cholesterol (under 50 in women, under 40 in men)

High blood levels of insulin

“Almost everyone can avoid type 2 diabetes,” Dr. Willett says. “Avoiding unhealthy carbohydrates is an important part of that solution.”

“They cut in front of fat as a fuel source, and insist on being burned first.”

But when you cut carbs, your body first fuels up with any available glycogen, he explains. When that’s gone, the body burns fat, a process called nutritional ketosis.

Eat 50 or fewer grams of carbs per day, and your body will have to use up fat. Meanwhile, your blood sugar levels stabilize and insulin production drops, Dr. Phinney says.

That’s why the diet works both for people with diabetes and for weight loss. Eating fat and protein increases satiety and reduces appetite and cravings, he says.

But eating simple carbohydrates causes an insulin surge, which triggers a drop in blood sugar, making you hungry again.

Low-Carb Resistance
But giving up carbs is hard to do.

That’s because they’re so tasty, and food manufacturers make them as tempting as possible, says Eric Westman, MD, director of the Lifestyle Medicine Clinic at Duke University Medical Center and co-author with Phinney of New Atkins for a New You (Touchstone).

Plus, switching to a low-carb lifestyle is difficult when potato-salad potlucks and bagels for breakfast are the norm.

But the biggest pushback against a low-carb diet comes from society, which has been trained to fear fat.

When you reduce carbs, you have to eat more fat to make up the calories needed to function. If a person needs 2,000 calories a day, and eats 50 grams of carbs, that’s 200 calories or 10% of his diet, Dr. Phinney explains. (Carbs and protein both have 4 calories per gram; fat has 9.)

That person has to make up the other 1,800 calories with protein and fat.

If he has 150 grams of protein, or 600 calories, that leaves 1,200 calories of fat.

“To stay on a low-carb diet, you must increase fat, which is an important nutrient,” Dr. Phinney says.

“But that’s still an uncomfortable notion for some people.”

Good and Bad FatsThough all agree that trans fats (the hardened oils added to some snack foods) are bad, at issue is which fats are healthy.

Some experts say any natural fat will do.

Others say stick to plant-based vegetable oils.

Dr. Willett’s advice: Go for unsaturated fats from plants, including vegetable, corn and peanut oils, which come as a liquid, and steer clear of solid saturated fats, like butter and other fats from dairy products and meat.

But Dr. Phinney says saturated fats are fine.

“People have long believed a link exists between eating saturated fats and increased levels of fat in the blood,” he says.

“That argument makes some logical sense, except it doesn’t bear out in science.”

In a 2010 meta-analysis published in the American Journal of Clinical Nutrition, the authors, including Dr. Hu, looked at 21 studies with 347,000 subjects and evaluated the association between saturated fat and coronary artery disease and stroke.

Their conclusion: “There is no significant evidence that dietary saturated fat is associated with an increased risk of coronary artery disease or cardiovascular disease.”

However, this license to load up on fat only works if you follow a low-carb diet, because the body’s ability to use fat as fuel goes up sharply.

Adding high fat to an already high-carb diet could be a deadly combination.

And the American average of 250-300 grams of carbohydrates a day is about double the USDA’s recommended dietary allowance of 130 grams.

Even some low-carb advocates believe that the limit is too inflexible.

The number of carbohydrate grams people can handle varies with age, metabolism, activity level, body size and gender, Dr. Phinney says.

For healthy adults, the number can be higher. Those facing insulin resistance do better on 50-100 grams a day, he says.

If you’re lean and active, you can eat more carbs than a fat, sedentary person, Dr. Willett says.

How to Get Started with Low-Carb EatingIf you just want to cut back on carbs, here’s how to do it:

How Well Do You Know Your Carbs?Can you tell whole wheat from wheat bread? Do you know which kind of pasta works with a low-carb diet? The best choice for a low-carb cocktail? It isn’t so simple to be carb-savvy. Put your knowledge to the test with this quiz to see how much you really know.

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